In a conventional 3-dimensional X-ray or cone-beam CT system, an X-ray source A, a sample D and a 2-dimensional detector B are disposed in one plane as shown in FIG. 5. The sample D is irradiated with X-ray from a direction perpendicular to (at 90 degrees with) a rotational axis C of the sample D. The projected image is received by the 2-dimensional detector B.
In such a cone-beam CT system, the X-ray generator or source cannot be moved sufficiently close to the rotating object due to its large rotation radius. Thus, even if one attempts to acquire a magnified image of a part of an object having a large surface area, a desired magnification cannot be obtained. In addition, when projection data on an object having a large flat surface area are acquired by irradiating the object with X-ray from a direction perpendicular to a rotational axis of the sample, beam hardening is undesirably generated in a direction parallel to the flat surface.
Beam hardening is described asunder. When X-rays having a sequence of spectrum penetrates through an object, low energy constituents appear to be attenuated more than their actual attenuation. This is believed to be because it is greater than the attenuation of high energy constituents. Because of this, when an image reconstruction calculation is performed where an effect of the spectrum is not considered, even a uniform substance appear non uniform. This effect is called beam hardening.
Japanese Patent Laid-Open Publication No. 2001-153817 discloses a laminography technique where an object is imaged from an oblique direction with respect to a rotational axis of the object. In such a laminography technique, several sets of 2-dimensional image data are generated. These 2-dimensional image data sets are then superimposed together 3dimensionally. Such a laminography technique enables one to make desirable observations in the 2-dimensional plane. However a clear 3-dimensional image can not be obtained due to insufficient resolution in height direction.
The disclosed teachings is aimed at overcoming some of the disadvantages in the conventional cone beam CT systems.